The present invention relates to an optoelectronic sensor for safe-guarding a hazardous area, and more particularly to a light grid having a first and a second sensor part which are arranged at a spatial distance from one another, wherein a plurality of transmitted light beams in opposite directions are formed between the sensor parts.
The invention generally relates to the field of safety technology in terms of protecting persons and/or material articles of value against hazards caused by machines or installations operating in automated fashion, such as e.g. robots operating in automated fashion. One typical protective measure consists in blocking off the hazardous area of such an installation, such that entry to the hazardous area either is prevented or leads to the stopping, switching off or other alleviation of the hazardous operating situation. Mechanical protective fences or protective doors are often used for blocking off such a hazardous area. However, access to the safeguarded hazardous area is often necessary, because for instance an operator requires access to the machine and/or because material has to be transported into or out of the hazardous area. Optoelectronic protective devices, such as light barriers or light grids, are often used for such cases. Such optoelectronic sensors generate one or more transmitted beams, which are detected by one or more receiving elements. If a transmitted beam is interrupted by a body part of a person, for example, this interruption is identified by means of the receiving elements and the installation can be stopped or switched off.
For such applications, it is important that the protective function achieved by means of the optoelectronic sensor is maintained even when the sensor has a functional fault or for other reasons is unable to detect and report the intrusion of an object into the hazardous area. Therefore, sensors for such applications are subject to specific requirements that are defined in particular in the European standards EN 954-1 and EN ISO 13849-1. On account of these specific requirements, a sensor for the applications described above generally differs from optoelectronic sensors for other, non-safety-relevant applications. Even though the present invention can in principle also be used in non-safety-relevant applications, it primarily relates to an optoelectronic sensor which satisfies at least the requirements of category 3 in the abovementioned standard EN 943-1 or comparable requirements with respect to intrinsic failsafety.
Light barriers, light grids and similar sensors typically have a transmitting part, which generates the transmitted beams, and a receiving part for detecting the transmitted beams. In many cases, the transmitted beams run parallel and in the same direction from the transmitter to the receiver. However, there are also concepts with transmitted beams in opposite directions between the two sensor parts, wherein these cases usually involve reflected-light barriers where a mirror in one of the sensor parts is used to generate a returning beam in response to an outgoing transmitted beam. A reflection light grid having such beams in opposite directions is known from DE 20 2005 010 358 U1, for example. Reflection light barriers and light grids have the advantage that only one sensor part has to contain active elements, while the other sensor part can be embodied in passive fashion. Cable connections can therefore be concentrated on the one active sensor part.
EP 1 615 053 A1 discloses a light grid having a modular construction and having a plurality of transmitter and receiver modules in both sensor parts, wherein a transmitter module and receiver module from each sensor part are opposite one another in such a way that the transmitted beam from one module can be detected by the receiving element of the other module. This arrangement is proposed, in particular, in order to be able to react to external light influences in a variable manner. Depending on the location of a disturbing external light source, only those transmitted beams which run counter to the external light source are used. This rules out a situation where the receiving elements are dazzled by the external light source. As is conventional in the prior art, EP 1 615 053 A1 proposes to connect the plurality of transmitting and receiving modules from each sensor part serially to a central evaluation and control unit.
DE 101 20 940 A1 proposes an optoelectronic sensor in the form of a light curtain, wherein individual light grid modules each are connected in series to form a first and a second sensor part, respectively. As in the light grid from EP 1 615 053 A1, the two sensor parts are connected to an external evaluation and control unit. Each light grid module generates a plurality of parallel transmitted beams running in the same direction. However, the respectively adjacent light grid modules are combined in such a way that groups of transmitted beams in opposite directions result overall. What is achieved in this way according to DE 101 20 940 A1 is that the adjacent light grid modules do not optically influence one another.
The previously known concepts for such optoelectronic sensors have the disadvantage that the reaction time of the sensor generally rises linearly with the number of transmitted beams used. Therefore, a light grid or a light curtain with a large number of transmitted beams typically has a longer reaction time than a light grid having a small number of transmitted beams. The reaction time of the sensor is an important variable because it determines the distance at which the sensor has to be arranged in front of the installation to be safeguarded. A large distance requires a large constructional space, which is disadvantageous for space and cost reasons. Shorter reaction times would be possible by means of a smaller number of transmitted beams. However, this would result in a reduction of the resolution, which in unfavorable cases could have the effect that a body part can pass through the optically monitored region of the sensor without being detected.
Furthermore, the previously known concepts for optoelectronic sensors of the type mentioned above are relatively expensive if a large number of variants for different protective field heights, resolutions and/or reaction times are to be provided.